1. Field of the Invention
The present invention generally relates to an automatic frequency control (AFC) in a mobile communication system in which the carrier phase varies. More particularly, the present invention is concerned with an AFC circuit capable of suppressing deterioration of demodulated data due to a frequency error of the carrier wave caused between the transmitter and receiver.
2. Description of the Related Art
There is a radio communication system in which voice communication or data communication takes place between a mobile station and a radio communication apparatus provided in a base station (hereinafter, such an apparatus is simply referred to as xe2x80x9cbase stationxe2x80x9d by a radio wave. If there is a frequency difference between the carrier waves on the transmission and reception sides, the quality of demodulated data will deteriorate greatly. Deterioration of the quality of demodulated data may specifically cause deformation of the signal waveform, or increase in the error rate of the received signal. Further, leakage of power to an adjacent channel may increase.
In order to avoid an occurrence of the situations mentioned above, an AFC circuit is conventionally provided in demodulation circuits of both the base station and the mobile station to correct a carrier frequency error.
When a frequency error caused in one of the transmission and reception sides is equal to an upper limit of a tolerable range and a frequency error caused in the other side is equal to a lower limit thereof, the frequency difference to be corrected by the AFC circuit is twice the tolerable range of the frequency error. The AFC circuit is thus required to have an ability of correcting the frequency over a wide range. When taking into account the above, the practical AFC circuit and a demodulation circuit including it have a large size.
In a radio communication apparatus in conformity with a personal digital cellular (PDC) system, a unique technique directed to correcting a carrier frequency error with a reduced load on the reception side has been developed. In this case, error correction is made on the transmission side in addition to the reception side on which error correction is passively made. A high-precision reference clock oscillator is provided in the radio communication apparatus. The AFC correction using the reference clock is made at the time of not only reception but also transmission. More particularly, the station detects a carrier frequency error between its own station and the remote station from the received signal. A control voltage corresponding to the carrier frequency error is produced and applied to the reference clock oscillator such as a voltage controlled-temperature compensated oscillator (VC-TCXO). The output signal of the reference clock oscillator is applied to a local oscillator on the transmission side, so that a local oscillation signal is produced. Thus, the frequency error of the carrier for transmission can be absorbed.
In practice, it is required to provide, in the base station, high-precision reference clock oscillators that are equal in number to mobile stations which may concurrently take communications and to simultaneously make frequency error corrections with respect to the mobile stations. The high-precision reference clock oscillators formed of VC-TCXOs are very expensive, and therefore raise the production cost of the base station.
It is a general object of the present invention to provide a radio communication apparatus and method in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a radio communication apparatus having a simple structure capable of transmitting a wave to be modulated having a relatively reduced frequency error in accordance to the other party.
Another object of the present invention is to provide a radio communication method capable of simply preventing degradation of the quality of decoded data.
The above objects of the present invention are achieved by a radio communication apparatus in which a received signal is demodulated using an output signal of a local oscillator, and the output signal of the local oscillator is modulated by a modulating signal including transmission information and is then transmitted, said radio communication apparatus comprising: a first unit detecting a frequency difference between the received signal and the output signal of the local oscillator; a second unit determining an amount of correction based on the frequency difference detected by the first unit; and a third unit applying the amount of correction determined by the second unit to the modulating signal.
A first radio communication method directed to achieve the above objects of the present invention is configured as follows. A radio communication method, in which a received signal is demodulated using an output signal of a local oscillator, and the output signal of the local oscillator is modulated by a modulating signal including transmission information and is then transmitted, comprises the steps of: (a) detecting a frequency difference between the received signal and the output signal of the local oscillator; (b) determining an amount of correction based on the frequency difference detected by the step (a); and (c) applying the amount of correction determined by the step (b) to the modulating signal.
A second radio communication method is configured so that, a received signal is demodulated using an output signal of a local oscillator, and the output signal of the local oscillator is modulated by a modulating signal including transmission information and is then transmitted, the radio communication method comprising the steps of: (a) detecting a frequency difference between the received signal and the output signal of the local oscillator; (b) determining an amount of correction based on the frequency difference detected by the step (a); (c) storing the amount of correction for every information unit; and (d) applying, to the modulation signal of a desired information unit, the amount of correction corresponding to said desired information unit.
The radio communication apparatus of the present invention can have various configurations. For example, the unit that acts as a correction unit may add the amount of correction to amplitude data or phase information. The decoding method may use differential decoding and any scheme other than the differential decoding. The present invention includes a communication which takes place in an information unit such as a slot in time division multiple access (for example, PHS or PDC) or a channel code in code division multiple access. The present invention includes the following radio communication apparatus.
A first radio communication apparatus is directed to adding the amount of correction to amplitude data, and includes: a first unit receiving a phase-modulated signal; a second unit extracting phase information from the phase-modulated signal received by the first unit by using an output signal of a local oscillator; a third unit detecting a deviation angle from a phase of a reference signal point of the phase information and determining an amount of correction based on the deviation angle; a fourth unit converting transmission data into a first pair of amplitude data corresponding to a signal point thereof; a fifth unit outputting a second pair of amplitude data corresponding to a phase obtained by adding the amount of correction to the phase of the signal point indicated by the first pair of amplitude data; and a sixth unit modulating the output signal of the local oscillator by using the second pair of amplitude data output by the fifth unit.
A second radio communication apparatus is intended to add the amount of correction to phase information, and includes: a first unit receiving a phase-modulated signal; a second unit extracting phase information from the phase-modulated signal received by the first unit by using an output signal of a local oscillator; a third unit detecting a deviation angle from a phase of a reference signal point of the phase information and determining an amount of correction based on the deviation angle; a fourth unit converting transmission data into phase information concerning a signal point thereof; a fifth unit adding the amount of correction to the phase information; a sixth unit converting the phase information with the amount of correction added thereto into a pair of amplitude data; and a seventh unit modulating the output signal of the local oscillator by using the pair of amplitude data obtained by the sixth unit.
A third radio communication apparatus employs differential decoding and adds the amount of correction to amplitude data. This apparatus includes: a first unit receiving a signal obtained by differential encoding; a second unit delay-detecting the signal received by the first unit by using an output signal of a local oscillator, so that an amount of phase shifting is extracted; a third unit determining an amount of correction based on an amount obtained by subtracting a reference amount of phase shifting from the extracted amount of phase shifting; a fourth unit outputting an integrated amount of correction; a fifth unit converting transmission data into a first pair of amplitude data corresponding to a signal point thereof; a sixth unit outputting a second pair of amplitude data corresponding to a phase obtained by adding the integrated amount of correction to the phase of the signal point indicated by the first pair of amplitude data; and a seventh unit modulating the output signal of the local oscillator by using the second pair of amplitude data output by the sixth unit.
A fourth radio communication apparatus employs differential decoding and adds the amount of correction to phase information. This apparatus includes: a first unit receiving a signal obtained by differential encoding; a second unit delay-detecting the signal received by the first unit by using an output signal of a local oscillator, so that an amount of phase shifting is extracted; a third unit determining an amount of correction based on an amount obtained by subtracting a reference amount of phase shifting from the extracted amount of phase shifting; a fourth unit outputting an integrated amount of correction; a fifth unit converting transmission data into phase information corresponding to a signal point thereof; a sixth unit outputting adding the integrated amount of correction to the phase information; a seventh unit converting the phase information with the integrated amount of correction added thereto output by the sixth unit into a pair of amplitude data; and an eighth unit modulating the output signal of the local oscillator by using the pair of amplitude data output by the seventh unit.
A fifth radio communication apparatus employs a CDMA system and includes: a first unit despreading a received signal from a party with which said radio communication apparatus communicates and detecting a deviation angle per pilot block due to a frequency error between the party and said radio communication apparatus by subjecting the despread received signal to a pilot synchronous detection; a second unit determining an amount of correction based on the deviation angle detected by the first unit; a third unit converting transmission data to be sent to the party into phase information concerning a signal point; a fourth unit adding the amount of correction to the phase information; a fifth unit converting the phase information with the amount of correction added thereto output by the fourth unit into a pair of amplitude data; and a sixth unit modulating a carrier wave for transmission by the pair of amplitude data output by the fifth unit and transmitting the modulated carrier wave to the party.
A sixth radio communication apparatus communicates with a remote party in an information unit, and includes: a first unit receiving, in the information unit, a phase-modulated signal from a party with which said radio communication apparatus communicates; a second unit extracting phase information contained in at least one information unit of the phase-modulated signal received by the first unit; a third unit detecting a deviation angle from a phase of a reference signal point of phase information concerning the information unit; a fourth unit determining an amount of correction based on the deviation angle detected by the third unit; a fifth unit storing the amount of correction determined by the fourth unit in the information unit; a sixth unit converting transmission data in the information unit into phase information of a signal point thereof; a seventh unit adding the amount of correction to the phase information in the information unit; an eighth unit converting the phase information with the amount of correction added thereto output by the seventh unit into a pair of amplitude data; and a ninth unit modulating a carrier wave in the information unit by using the pair of amplitude data and transmitting the modulated carrier wave to the party.